In general, a PN diode is used as an element configuring an ESD protection circuit in a semiconductor device which includes the ESD protection circuit. In addition, breakdown of the PN diode is used as a method of ESD protection in some cases.
Here, the PN diode which configures the ESD protection circuit is generally element-separated in the semiconductor device, i.e., it is isolated from the device it is protecting. As a method of element-separating the PN diode in the semiconductor device, there is a method of element-separating the PN diode using a dopant diffused layer. However, to form the dopant diffused layer, an extended time period of heat treatment must be performed. Therefore, the dopant is thermally diffused in the semiconductor layer, such that a distribution of a dopant concentration in a PN junction becomes moderate in some cases.
When the distribution of the dopant concentration in the PN junction becomes moderate, a breakdown voltage of the PN diode is unlikely to be lowered. Therefore, a clamp voltage of a protection circuit provided downstream of the ESD protection circuit is unlikely to be lowered.